1. Field of the Invention
This invention relates to a system for extracting plutonium from liquid phases, and more particularly, this invention relates to a system for extracting plutonium colloids and other oxides from spent nuclear fuel via liquid-liquid extraction comprising an organic and an aqueous phase extraction.
2. Background of the Invention
During the dissolution and reprocessing of spent nuclear reactor fuel, various phases of plutonium and other metals are formed, usually in the initial aqueous phase, i.e., the aqueous feed stock solution. One of the plutonium phases is referred to as “plutonium polymer.”
The problems associated with the formation of the polymers include clogging of stripping towers and liquid transfer lines. This creates criticality issues as well as the loss of material balance across plant operations. Ultimately, proliferation risks increase.
One solution to this polymer problem is to prevent formation of the polymer during fuel reprocessing. An alternative is to develop a solvent extraction system which targets removal and recovery of the plutonium fraction present in the polymeric phase.
Solvent extraction of plutonium polymer has been attempted with extractants such as tributyl phosphate (TBP). For example, Chaiko Separation Science and Technology 27 (11) pp 1389-1405 (1992), discloses a protocol for precipitating plutonium, with back extraction achieved using silica. However, the formation of precipitated plutonium raises proliferation and criticality issues.
Some extraction methods incorporate other organophosphorus extractants (e.g. phosphine-oxides). These methods typically extract all forms of plutonium, including plutonium-4, plutonium-6 and plutonium-polymer. Therefore, selectivity is nill. Also, these methods produce phosphate-containing secondary waste streams, which if incinerated, create environmental and final waste disposal issues.
Other extraction protocols eliminate the aforementioned environmental problems by using phosphate-free solvents. One such protocol is disclosed in Cuillerdier et al., Separation Science and Technology 26(9) pp 1229-1244 (1991). However, such non-phosphate systems lack selectivity and reversibility features.
In summary, state of the art plutonium extraction systems lack efficiency. Stripping of polymer is difficult, resulting in plutonium remaining in the organic phase. In situations where the polymer is successfully back-extracted from the organic phase, “crud” and emulsions form, neither of which is compatible with fuel reprocessing.
Most extraction technologies target molecular and/or ionic moieties of plutonium while not addressing colloidal plutonium. Molecular/ionic species of target metals consist of coordination complexes such as oxo-acid, halide, and aquated complexes, for example, Pu(NO3)4, Pu(NO3)62-, UO2(NO3)2 etc where only one metal ion is contained in the solvated complex. These species are originally generated in the aqueous phase by adjustment of the aqueous phase conditions. They are then extracted into the organic phase using the desired complementing extractant.
Colloidal or polymeric complexes on the other hand, consist of multiple metal or actinide atoms in the complex. They may coexist with molecular and ionic forms in the aqueous phase feedstock solution. This multiple atom feature of polymeric complexes presents challenges for metal extraction.
A need exists in the art for a system to extract colloidal plutonium whereby the extracted plutonium can be easily isolated for final disposition. The system should utilize relatively inexpensive reactants and result in no production of secondary waste streams. The system also should be compatible with existing plant operations, for example being operable in low (two molar or less) acid conditions.